The hypothesis is that specific serotonin reuptake inhibitors (SSRIs) may induce an abnormality in the secretion of antidiuretic hormone (ADH) and that this abnormality will manifest itself as impaired water excretion and/or hyponatremia in the setting of adequate or excessive water intake. The hypothesis will be tested by measuring serum sodium, osmolality and plasma ADH levels basally and in response to a sustained water load in elderly depressed patients treated with SSRIs. By administering a water load while the patients are treated with the SSRI, subtle adnormalities of ADH dynamics may be uncovered which otherwise may be overlooked in this population.